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Why do most glideslope transmitters bounce the lower beam off the ground, instead of simply transmitting it from an antenna lower down?
An ILS/IGS1 glideslope - the part of the system that provides vertical guidance to approaching aircraft and allows it to be used for a precision approach2 - works by transmitting two rapidly-pulsing UHF beams3 back along the approach path, one just above the desired glidepath and one just below it.  The beam just above the glidepath pulses on and off at a rate of 90 times every second (90 Hz), while the beam just below the glidepath pulses at a rate of 150 Hz; right at the correct glidepath, the two beams are of equal strength.  If the signal received by the aircraft’s glideslope receiver is dominated by the 90-Hz beat, this means that the aircraft is too high, and the pilot (or autopilot, if the aircraft is using autoland) needs to descend more steeply, while, if the signal received pulses more strongly at 150 Hz, the aircraft is too low, and the pilot/autopilot needs to slow their descent; the exact degree to which the signal is dominated by one pulse frequency or the other indicates how far the aircraft is from the glidepath.
With all glideslope installations, the upper beam is generated about how you would expect; the glideslope transmitter emits a beam of radio energy up and back along the approach path.  The process of forming the lower beam, however, is more complicated - the antenna array for this beam is angled downwards, toward the ground, which reflects it back upwards along the approach path:

(Image by the Federal Aviation Administration, via ymb1 here at AvSE.)
This imposes some restrictions on the siting of the glideslope transmitter, as the ground on the approach side of the transmitter has to be suitable for reflecting the lower beam back upwards - this requires a large area of fairly-flat surface in front of the antenna array, which can create problems if, say, the area near the end of the runway is far from flat, potentially making it impossible to install a glideslope or requiring that the transmitter be placed some distance to the side of the approach path (making the glideslope signal unuseable for the final portion of the approach).  Even if the terrain is suitable, the requirement for reflecting the lower glideslope beam off the ground creates a fairly-large area in front of the glideslope transmitter where the lower beam is very close to, or even touching, the ground, and anything of significant size and radiointeractivity which enters this area will interfere with the glideslope signal, making this entire area part of the critical area which must be kept free of vehicles and personnel whenever an aircraft is making an approach using the glideslope.
It would seem that a more robust and versatile system could be had if we were to do away with reflecting the lower beam off the ground, and instead simply transmit it up along the approach path in the normal manner, using a second set of antennae mounted lower on the antenna-support structure, or on a separate structure slightly farther back along the approach path; this would eliminate the need to worry about the UHF-reflectivity of the terrain in front of the antennae (since now the proper formation and aiming of neither beam would be dependent on its reflection from said terrain), and greatly shrink the size of the critical area around the antennae (instead of being a great long swath of ground where the lower glideslope beam approaches and meets the ground, it would be a much smaller area in the immediate vicinity of the antennae before the beams rose out of danger), so why does the standard type of glideslope rely on bouncing the lower beam off the ground?

1: An ILS (Instrument Landing System) is when it guides you right down to where you can see the airport out your front windows (visibility permitting); an IGS (Instrument Guidance System) is when it takes you to somewhere near(ish) the airport, but you’ve still got a bunch of manoeuvring to do after that (like that airport they used to have in Hong Kong).
2: Without the glideslope, the system reverts to a bare localiser (ILS) or LDA (IGS), which provides only lateral guidance (the aircraft has to use baroaltimeter or dGPS readings for vertical guidance), and, thus, is only good for a non-precision approach, which has higher minima (to help keep a misset baroaltimeter or GPS receiver from bringing the aircraft in short of the runway).
3: Glideslope transmitters transmit in a narrow range between 326.6 megahertz (MHz) and 335.4 MHz; the precise frequency used varies from glideslope to glideslope (to prevent an aircraft using one glideslope from being confused by signals from a different glideslope nearby), with each specific glideslope frequency being strictly paired with a specific localiser frequency (this allows a pilot to dial in just the localiser frequency - a specific VHF frequency in the 108-112-MHz band - and get the glideslope as well, without having to also manually enter the glideslope frequency).  Both the upper and lower beams of a single glideslope installation transmit on the same frequency.

The signal for the 90 and 150 Hz signals absolutely do not pulse on and off.  The glide path is produced using 90 and 150 Hz signals modulating a carrier wave.  The carrier with the resultant sidebands (CSB) is transmitted at the same time as a signal with just the resultant sidebands (SBO) (the SBO is further modified above and below the glidepath) the interference between the two generates the fly up and fly down indications.

The system you're after exists, but is a relatively recent invention
There are two styles of glideslope antenna in use today: the classical ground-reflecting "image" type glideslope that is described by the image in your question, and the alternate system you propose with a second set of antennas to generate the lower half of the beam, called an end fire system.  However, due to the presence of large amounts of relatively flat land at most major airports due to existing grading requirements, image type glideslopes sufficed for many years.
However, the safety benefits of precision approach capacity, or inversely the safety hazards of not having vertically guided approaches available, were a strong motivating factor for the airlines to push to have vertically guided approaches, such as ILS, expanded out to the fringes of their service networks.  This is what drove the development of the aforementioned endfire glideslope antenna, as they could work under more challenging conditions than an image type glideslope antenna could.
Note also that there are multiple flavors of image-type glideslope antenna (null reference, sideband reference, and capture effect), and some are more tolerant of poor site conditions than others.  Furthermore, even though endfire glideslopes can work in situations where an image-type glideslope cannot, they have their own limitations as well.  For instance, while well-sited image-type glideslopes can provide the tight tolerances required for a CAT IIIb ILS, and end-fire glideslopes can match or exceed this performance in theory, practically speaking, end-fire antennas are limited to CAT I use in the US by the FAA.
This is all documented in much more detail in FAA Order 6750.16E, for those who are interested in more detail than the overview I provided here.

While this post contains interesting information, it doesn't actually answer the question. We still don't know why ground-reflecting system was widely adopted instead of a "straighforward" solution. Apparently beaming both signals directly is not that straighforward as one might suspect and this is the core of OP's question.

Using the diagram in another answer, consider the vertical extent that would be required of the UHF tower if both upper and lower beams were made with physical transmitters.  This would be an unwelcome obstacle inside the protected area for an instrument approach.  Using the transmitter to bounce signals off the ground also neatly satisfies the requirement to add phase lag in the SBO signal that is required to produce the fly up or fly down signals.


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Why do most U.S. cities with multiple airports try to concentrate airline operations at just one airport instead of spreading out the load?
Many large metropoles around the world have multiple major airports with commercial airline service:

London, famously, has six (EGLL, EGLC, EGKK, EGSS, EGGW, EGMC),1 although the latter four are far enough away to seriously stretch the bounds of “serving London”.
Paris has two (LFPG and LFPO).2
Rome has two (LIRF and LIRA).
Moscow (the one in Матушка Россия, not the one in Idaho3) has four (UUDD, UUEE, UUWW, UUBW).
Istanbul has three (LTFM, LTFJ, LTBA [cargo-only]).
Beijing has two (ZBAA and ZBAD).
Shanghai has two (ZSPD and ZSSS).
Bangkok has two (VTBS and VTBD).
Seoul has two (RKSI and RKSS).
Tokyo has two (RJAA and RJTT).

In some of these cases (London, Istanbul, Beijing, Seoul), a new airport(s) has been opened explicitly to relieve the crush that existed previously, and/or preexisting satellite airports have been extensively touted as destinations in an effort to take pressure off overloaded primaries; in many to most of these cases, the existence of multiple airports has allowed particular categories of service to be concentrated at particular airports (for instance, one airport handles long-distance international flights while another handles domestic and regional flights, or one airport handles traditional carriers while another handles low-cost carriers, or [in Istanbul’s case] one airport is dedicated to cargo flights).
U.S. cities, like their international compatriots, frequently have multiple major airports.4 Unlike their international compatriots, however, U.S. cities show a pronounced tendency to try as hard as possible to squeeze everything into One Big Airport, with airlines often being kicked out of their homes at preexisting downtown airports and forced over to the One Big Airport (often an hour’s drive or more from the city it claims to serve):

New York has three major airports (KJFK, KLGA, KEWR);5 of these, KLGA, despite being the most centrally located, has been consistently discriminated against in favour of the Two Big Airports (KJFK and KEWR), being almost entirely barred to flights to or from destinations outside the eastern half of the United States, and completely barred to flights operated by non-North-American airlines.6
Chicago has two major airports (KORD and KMDW);7 despite KMDW’s closer-to-downtown location, the vast majority of traffic goes through KORD, with only one airline having more than a token presence at KMDW (that one airline accounts for over 95 percent of all passengers using the airport).
Dallas/Fort Worth has two major airports (KDFW and KDAL); almost all traffic (again except for essentially just one airline) uses KDFW, which is centrally located for neither city.  In fact, for many years, airlines were barred by federal law from operating full-size flights to or from airports not located in Texas or a Texas-neighbouring state out of any Dallas/Fort Worth airport other than KDFW,8 in a deliberate attempt to force everyone to use KDFW; this nearly caused the end of airline operations at the much-more-Dallas-convenient KDAL (which was only saved following a lengthy court battle), did, in fact, spell the doom for what was then Fort Worth’s main airport (KGSW, which was used solely for airline training flights after 1969, closed entirely in 1974, and almost completely razed in the 1980s and 1990s), and prevented KFTW, very close to downtown Fort Worth, from ever becoming successful as a commercial airport.
Kansas City has two major airports (KMCI and KMKC); nearly all airline traffic goes through the One Big Airport (KMCI), located far from the city, while the centrally-located KMKC sees only limited cargo service.

(Washington, with two major airports [KIAD and KDCA], is a rare exception, and, even here, they tried [albeit incompletely successfully] to shoehorn everyone into One Big Airport; KDCA, located almost directly across the Potomac from downtown Washington, sees approximately the same level of service as KIAD, despite the former being restricted [with only a few exceptions] to flights no longer than approximately 2000 kilometers [in an effort to force flights to use KIAD, located nearly 50 kilometers away from the city].)
Why do U.S. cities have so much of a tendency to try to shoehorn everyone and everything into One Big Airport (or, in giant New York’s case, Two Big Airports), rather than spreading out the load between multiple smaller airports?

1: Two others (EGKB and EGWU) are kept barred to airline service due to selfish NIMBYs.
2: A third, LFPB, was closed to commercial traffic in 1980 (no reason given, but I’m suspecting it’s the damn NIMBYs again).
3: Which is itself served by an airport!
4: Possibly even more frequently, given the quintessentially American love of aviation.
5: A fourth airport, KHPN, sees some airline service but is handicapped by draconian passenger-volume limits (NIMBYs again) that keep it far below its full potential, while a fifth, KTEB, is completely barred to commercial service via NIMBY-imposed weight limits.
6: In fact, when KJFK (known at the time as KIDL) opened, the European airlines that had, until then, been happily resident at KLGA were forcibly evicted to the new, far-away airport through the cancellation of their permits to operate from KLGA.
7: A third airport, KCGX, located right on the downtown waterfront, was forcibly closed by the mayor of Chicago in 2003 because he didn’t like it.
8: These restrictions were loosened considerably in 2006, and finally repealed entirely in 2014.

In the Istanbul and Beijing cases, the new airports were opened with purpose of MOVING the traffic over there from the original airport, not splitting it. And there are places like Berlin that used to have three airports, have two, and is trying to concentrate it to only one.

I find the use of the term 'selfish NIMBYs' quite inappropriate. Everyone deserves to have their views heard and the quality of life respected. It's appropriate to consider those in the light of a wider requirement and act accordingly. In the (now unlikely) event that Heathrow's third runway is built the residents of Harmondsworth can expect to have most of  their village demolished. Would you consider them to be 'selfish NIMBYs'?

I would suggest that a large part of the answer is simply connecting flights.  Bad enough when you have to run through multiple wings of a terminal because your inbound flight was a bit late, imagine trying to get from Idlewild to Kennedy, or LAX to Orange County, at the height of rush hour.  And then there's your luggage...

@jamesqf Idlewild IS Kennedy.

@TomMcW: My mistake.  I think I meant LaGuardia?  I do live on the other side of the continent, so not really familiar.

NIMBYs for Le Bourget? neighbors loved so much the airport that they built a lot around it, up to the point it was not anymore safe to operate flights from there.
I guess you would have nothing contrary to me purchasing mineral rights below your house and drilling a borehole or a mining pit in your basement to provide resources to our community, right? or are you one of those selfish NIMBYs people?

Amusingly, London now "has" seven airports.  EGTK, 56 miles/90km from London, styles itself "London Oxford Airport".  Won't be long before my local airport EGNT is also called "London".

A successful airport needs five things to thrive:

Financially solvent airlines/aircraft operators
Paying customers or passengers
Federal government support
Local popular support
A strong local tax base

In the case of Dallas, this consolidation was necessary to get KDFW built. Before that, Dallas Love Field was considered Dallas’s main airport and Greater Southwest International Airport was Fort Worth’s main airport. But, it took a combined effort of Dallas, Fort Worth, the Mid-Cities, and their five associated counties to appropriate the land, as well as tax and municipal bond dollars necessary to fund the project. That is why it was negotiated to put KDFW in the central-most location to the cities in the metroplex.
It was also a requirement from Fort Worth (the smaller of the two main cities) that Love Field and Greater Southwest International Airport not pose a significant threat of competition to the joint KDFW airport. Greater Southwest was closed as an airport. And, Love Field was put under legal restrictions curtailing operating flights with destinations outside of states adjoining Texas. This law was called the Wright Amendment. Fort Worth was allowed to retain a much smaller airport called Meacham International Airport.
The primary reason for this consolidation was two fold. One of which being the vast geographic range of the DFW Metroplex area. An airport on one side of the Metroplex could not effectively service the other side of the Metroplex. Operating multiple large airports puts an undue financial burden on the FAA that subsidizes international and national airports in the US. Other Class D, E (to the surface) and G airports around the DFW metroplex get much less funding from the FAA or none at all. They draw the majority of their funding primarily from private, corporate, or local municipality sources.
The other side of the coin is that airlines are able to consolidate their resources at the consolidated airport. Airlines would not have to sacrifice servicing half of the metroplex in order to reap the benefits of servicing the other half. And, neither Love Field nor Greater Southwest had the room to grow. Like Midway, the runways for those two (and still for Love Field) are comparatively short. And, except for the North end of the runway, the entire Love Field airport is closely surrounded by long-standing residential neighborhoods. Just like Midway.
As far as Chicago, it has been many years since I lived there. But, as I recall, it bears the same geographical problem as DFW. O’Hare Was supposed to completely replace Midway Airport. If not for a few discount seeking airlines and corporate air operations, Midway would have dwindled down to a Class D airfield.
O’Hare’s location is the central most location for servicing an area as far East as Gary, IN and as far West as Rockford, IL. Both of these cities have their own airports. But, there is not enough traffic (passenger business) to support major airlines. And, there is not enough of a tax base to generate major funding. This leads the two respective airports to serve as feeders for the main O’Hare hub. Most people (from my memory) preferred to just drive into O’Hare from Gary and Rockford to catch their flights. You can’t force the airlines to lose money by servicing an airport that does not generate revenue. And, you can’t force the local population to buy airline tickets too expensive to be worth the convenience of not driving to a busier airport. Case in point, Rockford lost all passenger service for 2 or 3 years in the early 2000s.

This puts too much emphasis on GSW. FTW had commercial service for decades, and there were even flights between FTW and DAL. But the airlines wanted a single airport. GSW was the first attempt, but it wasn't big enough, so DFW was built to replace GSW.

@StephenS - While what you say is true about Meacham, it was not Fort Worth’s major player when KDFW was being built. If GSW were better planned, we would still be having the same conversation, replacing KDFW with KGSW in the questions and answers. Consolidation was what everyone with real money and clout wanted (airlines, FAA, etc.).

Paris is a good example because it has multiple major train stations that work the way you describe: traffic is distributed between them more or less equally, with each of them serving a certain geographic region of France in a hub-and-spoke model just like airlines do. Which means that if you want to go from one region of France to another, you always have to take a train to Paris, then leave the train station to get into the city, take the métro / a regional train / a bus / a taxi to get to the other train station, then get onto the next train.
This is very annoying and it would be much simpler if there was only one train station and you could simply change trains there without also having to change stations.
The same applies to flights. Everybody who has ever arrived at JFK and had to catch a connecting flight at LaGuardia during rush hour can attest to that.

FWIW London mainline train station work the same way.

In many European cities, railway lines were constructed in a hub-and-spoke model. This goes so far, that some stations are named for the general direction of the train line, e.g. Gare de l'Est serving the region east of Paris; or the terminus at the other end of the railway line, e.g. the Leningradsky station in Moscow. However, nowadays there is a push away from having multiple stations terminating at several points within the city, instead interchange stations are preferred.

It would not necessarily be much simpler if most of the trains had to go halfway around the city to get to a single humongous train station, which would be super-packed.

The Paris example is famous enough to have become standard terminology in mathematics.  https://math.stackexchange.com/questions/783567/how-to-prove-that-the-french-railways-metric-is-a-metric

As far as Kansas City’s situation, MCI was built to basically replace MKC. Larger airliners had simply outgrown the old airport. They needed space for runway safety areas and there was no room to expand in the middle of the city. Also the approaches are tight at MKC with high bluffs and tall buildings just to the southeast of the airport. According to Wikipedia, in 1963 the FAA called the downtown airport "one of the poorest major airports in the country for large jet aircraft" and recommended not to spend any more federal money on it. A 1965 runway overrun by a 707 may have been the final nail in the coffin for MKC as the primary airport. MCI was built out in the country where there was more land available for expansion.
Another consideration for downtown airports is noise. An airport being located closer to a city center may make for easier access, but it’s a nightmare for noise ordinances. And if MKC were still the main airport I can’t imagine what kind of mess adding all the MCI traffic to downtown would create.
There was an idea floated a few years ago about building a rival airport on the Kansas side, but it never got past the rumor stage. The discussions in Missouri about the new terminal at KMCI were getting messy and the Kansas governor mentioned the possibility, but gave no details. Johnson County, Kansas already has two Class D airports, KOJC and KIXD, but neither handles any scheduled airline traffic. OJC has been hemmed in by housing so it is not suitable for expansion. IXD has enough room, but I have a hard time imagining Johnson County residents being OK with all the increased noise of airline traffic.

Nowadays, KMKC would very likely be safe for jetliners if EMAS beds were installed; unfortunately, EMAS hadn't yet been invented in 1965.

I will only speak for the USA.(but we do have the majority of the worlds busiest airports) Most airports in the US are operated on behalf of local cities. The political structure of our city level governments produces poor long term planning. For multiple Airports to work as a unit you need a direct rapid and reliable transit between the local airports without the encumbrance of the travel security circus. Such a transit cannot rely on public streets, thus it requires the use of land and while many want to use the service few are willing to tolerate it near their own land. In addition flight is still seen as a service for the wealthy and is no longer viewed as travel of the future, so they do not pull large numbers of voters, thus airports are under much pressure for budget and local land use restrictions. I do not know of any new airports being constructed in many years, even where there is apparent need, just expanding runways for safety takes years of political fighting. All of which combines to prevent the needed connections.
There is federal grant money for airports but it comes with so many strings attached that the airports who accept the money can hardly be operated as proper businesses for decades after and beside that the money is narrowly directed on airport and could not be used to connect separated airports. However most airports must accept the money with all the strings because it is the only way to pay for every increasing government requirements such as remodeling terminals for TSA screening. (while loosing massive business revenue because of the same checkpoints)
And so, domestic hub and spoke airlines have been in decline and point to point have been on the rise for a couple of decades. This can increase use of many second tier airports without the need to connect them. There are no more domestic passenger 747 or A380 operating in the USA because of this, most hub based airlines and airports are used for connecting with international flights.

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Why didn’t the Me 163 experience the serious handling problems suffered by other similarly-designed semitailless aircraft?
Although the Messerschmitt Me 163 rocket-powered interceptor had a number of frequently-lethal features (its extremely-toxic, violently-reactive, enthusiastically-explosive hypergolic propellants; its few-minutes-brief endurance in powered flight, allowing enemy fighters to wait and then pounce on it as it glided home; its skid landing gear, which rendered it an immobile sitting duck upon landing until it could be towed to safety), its handling characteristics were not one of these; despite its unusual semitailless design (a vertical tail, but no separate horizontal tail), it was extremely docile at both low and high speeds (despite its excellent manoeuvrability, something more usually found in conjunction with very un-docile handling), being difficult to stall or spin even deliberately and showing no untoward changes in stability characteristics even at very high speed.
In contrast, other similarly-configured semitailless aircraft (DH 108, X-4, F7U) were uniformly beasts to fly, being prone to severe pitch oscillations at high speed, yaw and roll oscillations as well at slightly higher speeds, and generally poor handling characteristics overall.
What kept the Me 163 free of the handling difficulties that plagued all those other semitailless aircraft?

Firstly, the Me 163 is classed as tailless, not semi-tailless. The fin does not count.
Secondly, its characteristics were not wholly benign but they were acceptable by the standards of the moment for a high-speed fighter. Its lateral stability was marginal and it could behave unpleasantly in the stall. When Junkers developed it further, they found it necessary to make the leading-edge slots automatically variable and to increase the area of the landing flaps. But on the whole, yes, it was better than most tailless types.
So, why was this particular plane adequate? You will have to bear with me. A long trail of aerodynamic refinement begins with JW Dunne's tailless swept monoplanes and biplanes of the pioneer era. His D.10 biplane became the first aircraft ever to be officially certified as stable. The finless monoplanes could at least be stalled, but simply pancaked down rather than dropping a wing. Around thirty Dunne machines were flown in the UK and US and none ever had a fatal accident, a staggering safety record for that era. Dunne's secrets were many and subtle, expounded to the Aeronautical Society of Great Britain in 1913.
In the 1920s an ex RAF Captain, GTR Hill, took up the unstallable challenge once more and refined Dunne's aerodynamics, developing among other things the mathematics of the constant centre of pressure via reflex camber. A German visitor whose name I forget took it back to Germany where it was further enhanced and there Alexander Lippisch discovered it. He had the luxury of ten years' single-minded research and experimentation on one prototype after another, from glider to propeller to rocket, until the Me 163 emerged. But neither Hill nor Lippisch could get rid of the fin.
Meanwhile the detailed technical work of Dunne received only voice plaudits from others, while that of Prandtl in the early 1930s went unnoticed. Reimar Horten trod the path for himself to arrive at the finless Horten/Gotha flying-wing fighter which also handled well but came too late for wartime production.
What Dunne intuited and tested in hundreds of models, Lippisch half-grasped and Horten rediscovered and calculated, never made it over to Northrop or General Aircraft or any of the others. When Dunne's trademark conical leading-edge droop was rediscovered and used on the Convair deltas to make them flyable at low speeds, the path to it lay through quite another labyrinth. Back in the UK, Dunne and de Havilland renewed their old friendship, for DH had been Dunne's successor at Farnborough and both had trod the corridors of the RAeS together. DH wrote to Dunne that during the development of the DH.108 he would tease his staff that Dunne had got it right all those years ago, so they better had too. But like Lippisch, even he had to keep the fin. And despite also being informed by both Lippisch and the Hortens, his DH 108 was described by one test pilot as "malignant".
The real questions must be; why did so many designers turn a blind eye to the tailoring of span loading and sideways flow to eliminate adverse yaw and the need for a tail fin? and how did they manage to introduce appalling stall characteristics when Dunne's original breakthrough was to tame the stall? The key lessons had been in print since 1913, for those who had eyes to see, and Horten at least had proved capable of finding them for himself. Simple ignorance, preoccupation with conventional design issues and "not invented here" are my best guess. Prandtl's and Horten's contributions are only now beginning to receive wider acknowledgement. The epilogue must go to General Aircraft, whose GAL.56 was described by the ubiquitous Winkle Brown as the most badly-behaved aircraft he had ever flown, before it went on to kill the most experienced tailless aircraft test pilot in the world and who had consequently flown many Lippisch creations, Robert Kronfeld.
For those who want provenance and more detail I fear that you will have to wait until I can get quite a backlog of stuff past our Covid-coma'ed publishing industry. Or you may tread the Science Museum Archive's Dunne Collection for yourself, all 30,000 documents and boxfuls of artefacts of it.

Was Otto Lippisch in any way related to Alexander Lippisch? Strange that both designed tailless aircraft at the same time …

@PeterKämpf Well spotted, now corrected. I made that same typo a few weeks back. I shall be embarrassed if it was here and you were the one who pulled me up then!

"Thirdly, the DH.108 was also a pleasure to fly, better in fact than the Me 163 (Eric "Winkle" Brown was among those blessed few who flew - and survived - both)" - Would this by any chance happen to be the same Eric "Winkle" Brown who described the DH 108 as "A killer [...] Vicious undamped longitudinal oscillation at speed in bumps." and the Me 163 as "the only one [of the five types of semitailless aircraft he flew] that had good flight characteristics"?

@Sean You are right. The "killer" epithet was largely because of the flutter at high Mach number, a problem not confined to tailless types. But on re-reading his accounts it was unpleasant in other ways, even "malignant". I will delete that bit of my answer.

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How can a headset cause continuous transmission (PTT blocked)?
I own three aviation headsets:
- BOSE A20
- David Clark H10-13.4
- Compilot Lite (a cheap headset bought recently, for occasional use by passengers)
I tested the Compilot Lite last week (co-pilot side), but I ended up blocking the frequency at a local airport. Unfortunately, it took me some time, and somebody waving furiously at me, before I found out.
I was wondering how this issue could occur and what could be wrong with the headset. I found an explanation on this forum, which states that the tip of the connector functions as a PTT button when grounded. However, the connector does not seem to differ a lot from the other headsets.
Left = Compilot, middle = DC, right = Bose.

Could it be caused by something else? The type of microphone?
Any tips or suggestions are welcome.
Thanks!
David
PS the aircraft was a C172N, a rental which I often use, but never had this issue before with the other headsets

@mins the one on the right is a Switchcraft mil- spec M642/5-1. It is a 3 conductor plug. The others are generic PJ-068 connectors. They’re supposed to be compatible with each other.

Could be a wire broken or grounding out somewhere past the jack itself.  The headset might be fine to use as designed, but with a broken/grounding wire, it ends up being a continuous hot mic. I flew with a pilot several years ago whose headset would occasionally create a hot-mic because its wires were so worn out, and of course we were the only ones who couldn't hear it. Perhaps your headset is doing something similar, but continuously. Maybe an avionics shop could take look at it?

@TomMcW: You must be right. Searching a bit I understand that the Bose has a 6-pin jack and needs this adapter to get usual jacks, of which the one in the post.

@mins That’s a LEMO connector, which is an option on Bose products. But the connector he is showing is the .206” phono connector. The A20 can be ordered with either type

@TomMcW "the one on the right is a Switchcraft mil- spec M642/5-1. It is a 3 conductor plug." Are you sure? It looks an awful lot like a TRRS plug to me, not at all like a TRS. The Compilot and the DC are squarely TRS though, and since the OP's issues are with the Compilot but the DC works fine, it stands to reason that this isn't the cause of the issue.

@MichaelKjörling Yes, the mil spec connectors have an extra piece of metal between two halves of the insulator between tip and ring. see drawing

The headset was really cheap, so I guess I got what I paid for :-) Either way, I have now learned something about the connectors and how the PTT button functions.

The issue has been found ... the headset cable was damaged. One of the audio cables was slightly exposed, causing it to touch the ground (which is the shield of the cable and is not covered). This only causes a continuity when connected to a radio, so it was hard to find ...
Kind regards,
David

Good that you found the problem. I guess that goes to prove, at least to some extent, that you do indeed get what you pay for.

Apparently the tip connection is being grounded activating the PTT. From the photo the only thing that could possibly be a problem is the Compilot plug being a tiny bit longer than the DC. But the detent in the tip is what should decide where the plug sits in the jack and that looks fine. It almost has to be a wiring problem in the Compilot. If you have a multimeter check for continuity between the barrel and the tip. There should be no continuity unless a PTT button is being pressed.

Thanks for your feedback! I have used a multimeter, but could not find a continuity on the plug. I also connected this headset to my handheld radio (iCom) and after moving the cables around, the issue was there again. So, indeed a fault in the wiring .. I guess it is Murphy who makes it harder to find when not in an airplane :-)

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text:
On SEP airplanes, why does the fuel consumption decreases with temperature, pressure and airspeed being equal?
Looking at the cruise performance tables in the POH of the C150, C172 or C182, one can see that for the same pressure altitude, the same RPM and the same true airspeed, the fuel consumption is reduced at higher temperature.
For example, a 1976 C150 POH indicates, at 2500 RPM, at 2000 ft pressure altitude,

at -5°C: 97 KTAS, 5.3 GPH
at 15°C: 96 KTAS, 4.9 GPH
at 35°C: 95 KTAS, 4.6 GPH

(yes, there is a 1 kt difference each time, but interpolating the values gives very similar results)
But thermodynamics tells us that the efficiency is proportional to the difference of temperature between the isothermal expansion at Th and the isothermal compression at Tc in the Carnot cycle, thus efficiency = (Th-Tc)/Th.
I cannot reconcile the consumption data with that theory, since apparently the output power is the same (true airspeed, with identical parasitic and induced drag forces) for an apparently higher efficiency at higher Tc temperature.
What other factor am I missing?

true airspeed, with identical parasitic and induced drag forces Why would the drag be identical? Same TAS in lower density air means lower IAS and less drag.

In the examples cited, there is a slight increase in Density Altitude (DENALT), which results in a slightly higher TAS.  As the air becomes thinner, fewer molecules pass by the aircraft per mile, resulting in a lightly less loss due to drag.  Increased temperature does not have the same range of change as altitude, however, it is nonetheless a higher DENALT, which is thinner air.
As an aside, in practice, you will note that the throttle becomes wider open with less dense air, until it is fully open. (Note that RPM is not the primary measure of power, rather fuel burn is. RPMs will become higher at higher DENALT for the same power.) At that point there will be no more available power, and any increase in temperature or altitude cannot be compensated for by increasing the throttle setting.
You will also note that less energy is expended per mile at higher DENALT, not factoring in the cost of a climb to that altitude.
Finally, a different practical example of this: When flying in icing conditions, conditions (and the POH) may call for continuous carburetor heat at altitude. When this is done, the engine should be releaned, as the DENALT will be higher, and if uncompensated for, will result in a rich mixture which could foul spark plugs.

Very clear and thorough explanation, thanks! I was neglecting the impact of temperature on parasitic and induced drag (as others also helpfully remarked).

As the temperature increases the air expands becoming less dense. In a normally aspirated engine, the less dense air means less oxygen is drawn into the cylinders and less fuel is required to maintain the proper air/fuel ratio.  Thus, fuel consumption is reduced.
This also results in less power and a lower IAS. But the higher temperature helps offset that resulting in only a small decrease in TAS.

Exact, thanks a lot!

The drag is also another important factor.

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text:
Does a long time to solo hurt your chances of becoming an airline pilot?
I've heard that on job applications for airline pilots that they ask the question: "How many hours to Solo?".  If it took someone 30 hours to solo but they had more logged flight time than someone who took 10 hours to solo, would the person who soloed in 10 hours have an advantage?

Generally they try to find the reason behind the numbers.

Doesn't it depend on the school you train with? It's kind of in their financial interest for them to be slow to let you solo, because of the margins on instruction and because they're there to build ATP hours. Is that overly cynical and not something people in industry take into account?

The honest answer:  nobody cares or would even ask you that question during a professional pilot interview.  Aside from a military pilot job where they can wash you out during pre-solo training, it doesn’t matter.  Primary checkride failures can be dings against you if you have a long string of them without a solid explanation.  But everyone fails a primary checkride now and then.  Now they will care about initial and recurrent type rating checkride fails for part 121 work.
There are a couple of exceptions these days.  Many Chinese airlines who send new pilot recruits to flight schools in the states set time limits for soloing.  Exceed those and you will not become a pilot with them (that and you’re obligated to work in another role within the airline for ten years or so) but the majors or regionals here in the states?  They don’t care.
What will kill an airline career are:

Part 121 checkride failures
DUIs
Felony or drug convictions
Bad driving records
History of aircraft accidents or incidents
FAA violations, certificate suspensions or revocations
Cheating on FAA exams or logbook doctoring or
Lying about any of the above on a job application.

Basically the airlines want to see that you’re a safe, trustworthy, reliable individual at the controls of an aircraft or other motor vehicle.  They’re also looking to see if you can fit into company culture and fly the way they want you to fly.
Similar to school, the airlines look to your last employer.  Each subsequent increase in responsibility will be what matters.  If you have 3000 hours PIC in a ERJ175 with a good reputation and references from the firm you work for, do you think they will really care that it took you 30 hours to solo in a 172?

It depends on the final product presented to the airline.  If an airline had two candidates, one of whom soloed in 10 hours and did well in a Sim Eval but was self absorbed, didn't like to follow procedures if he/she knew better, and didn't work well with people, and a candidate who took 30 hours to solo but was a team player, can get along with anyone, was an eager learner and was emotionally mature, and did a passable job in the Sim Eval, they'd take the "slow learner" in a heartbeat if they were confident the candidate could get through the type course.
In the airline world, raw flying skills are an important baseline, but big picture personality related issues are just as important and personality problems with crews are a bigger long term problem than how long it took someone to catch on at the start.

Good answer.  It has been a few years since I filled out an airline app, but I don't remember this question coming up.  I think that by the time someone gets to an ATP the playing field is a bit more level though.

And one of the most important skills is being able to make friends, and especially, avoid making enemies.  Even in the US it is a small world and you never know when the guy you annoyed once turns up as the chief pilot at the operation you're applying for years later.

Extrapolate this into any career. I'm a developer and EQ is not exactly.. high in our field. If I was hiring, I would pick an average programmer with high EQ over a great programmer with no social skills every single time.

The interesting thing is the different interpersonal factors with airline vs corporate.  Airline crews only have to get along with each other for a few days at a time and may not see each other again for months.  Corporate crews may fly together all the time so long term relationship skills are more important.  In a corporate flight department, "fitting in" at a personal level is critical and is one of the major sources of stress for a new hire.

Great question as it illustrates how the job applicant can improve their chances of employment by knowing what the employer wants.  Years ago some flying schools were advertising 10 hours to solo courses.
I took 40, but had no previous flying experience.
The major concerns were thoroughly knowing the slow
flight characteristics of the plane, building confidence, learning and following landing check lists, handling cross winds, and go around procedures.
While it seemed easier to grease a plane in modulating
the throttle, I was always worried about the turn base to final under power.  More time was taken learning to
come in high and land by adding drag to the plane with flaps and forward slipping.  This seemed better because the runway could always be made. I also liked it because it was similar to the engine out emergency landing procedure.  Everybody has their own comfort level.  But now nearing retirement,  my advice would be to not worry about what is on the resume, but how you would answer a question about it.  "I took the time to practice until I could safely do it" might be worth considering.

Although they give some good information, neither of the two answers actually answer your question.

would the person who soloed in 10 hours have an advantage?

Yes. All other things being equal, soloing after 10 hours shows on paper you are either a faster learner or just a more focused learner to me if I was looking at potential job applicants in a hiring manner. John K makes the point there are other factors we look at too (obviously) but to answer your question, 10 hours is more impressive than 30 hours (this kind of difference is significant).
It's like college GPA on your resume. Yes extracurriculars, and how well you get along in the interview is definitely important, but if two candidates are pretty much identical in all other factors, why would anyone choose the lower GPA candidate.

Problem is, two candidates (or human beings on the planet) are never identical. If they seem that way to you - you missed something.

"a faster learner or just a more focused learner " - Not necessarily. 10 hours might just show you were trying to save money by cutting corners in your training, you chose a cheapskate flying school that would go along with that attitude, and you got lucky enough not to crash.

@alephzero. I 100% agree it could. Just like a higher GPA could mean you chose a cheapskate uni with easy subjects in courses which curved, and got lucky that the grade got curved so most people got A's. Again to answer his question though, in general, all other things being equal, on average, someone learning to solo in literally a third of the time as someone else usually leads me to favour the one who learnt faster.Maybe some % of the time I'm choosing the cheapskate (and the interview/past checkrides etc. should pick this up) but on average its an adv and thats what the question was asking

A lot of these answers are talking about "it depends on other things" or "maybe the 10 hour student wasnt adept just lucky but the 30 hour student wasn't clumsy but just cautious" which all are 100% potentially true and legitimate concerns, but that's not what the question is asking. He's not asking how much of an advantage 10 hours is to 30 hours (which I agree should be very little especially compared to other factors like if you have 4+ checkrides etc.) he's just asking IF there is an advantage. And if you learnt to solo in 3x as long, that's more often a disadvantage than an advantage.

If an airline pays too much attention to such difference, I would be worried. Very few accidents are caused by improper handling of the controls;  the majority are due to poor pilot's decisions and discipline, so I expect airlines to focus on the latter.

It's not a factor, to be honest, unless your time to solo was extraordinarily long. Solo endorsements vary a lot based on the instructor in addition to the student. You could have an overly careful CFI that won't let you solo until 30 hours, or maybe you have some crazy cropduster type that'll sign you off at 10.
If an airline is comparing two pilots that have very similar backgrounds, and they start comparing times to solo, they will surely take a step back and figure out a way to see each pilot from a bigger picture perspective. I am speculating on this, but I see a major airline asking an extra personality question before looking at time to solo.
One guy said time to solo is like GPA on a resume. It's more like a fine detail on your transcript showing you took Math 1, 2, and 3 while the other guy skipped 1 and just took 2 and 3. Sure, he was better at math than you at one point, but you could also argue that you understand the underlying concepts better than him.
Don't fret about time to solo. Fret about which tie you're going to wear.

Depends where you learn to fly as well. Having a 1682 foot home field with obstructions at both ends is a bit harrowing to learn at vs a 5000 foot field that is wide open for instrument approaches.  After learning at a 1682 foot field, with no shot at putting down near the numbers, everyplace else just seems unnecessarily long now :)


text:
What could cause the Boeing "Pickle Fork" issues?
The Boeing 737-NG has been in the news recently regarding cracks discovered in the "pickle fork" assembly which transfers loads from the wings to the body of the aircraft.  The "pickle fork" was supposed to last the lifetime of the air frame (90,000 cycles), but cracks were discovered in airplanes with less than 30,000 cycles.  Could the cracks be due to an "over G" situation where turbulence or a hard landing could have caused the airplane to exceed its load limit, or is this likely due to a design flaw?

@blacksmith37 the name refers to the shape of the part, not the cracks.

Depends on the crack characteristics, number of striations (individual propagation events) etc., whether the cracks are from a small number of near-yield overloads or many small loads from regular cycles.
This sort of thing happens quite a lot.  You are designing parts at all times to be "just strong enough" for all the various requirements and goals, since every pound over that is considered ballast.  Computer finite element analysis makes it possible to fine tune the structure during design and reduce the fudge factor needed in the slide rule days.
Endurance fatigue testing of the part is used to validate the finite element analysis. A 3:1 safety or scatter factor is applied in the fatigue endurance test.  To be certified for 90,000 cycles, the part had to last 270,000 cycles crack free in the fatigue rig.
As a back stop to all that, you have a structural inspection program to cater to the possibility that the scatter is bigger than expected or the analysis was flawed and wasn't revealed by the original endurance test.  Everything has variability, generally on a pareto distribution (bell curve).
If you only endurance test one or two articles, you can't really be sure where those two parts would fall on the bell curve.  You assume it's near the middle, and the inspection program covers your butt if you are wrong.  Which is the case here probably.   You could test a large number of forks instead of a couple to reduce the scatter factor risk, but $.
You'd need to see the lab reports for the Pickle Forks (I've read many).  There may have been microscopic flaws in manufacture (pits or nicks or voids in a forging) that created crack initiation points.  It  may be a batch issue of a defect like that on x number of parts.  Or the original analysis was off, and the fatigue testing got units that represented the right side of the bell curve (low probability) and gave an optimistic result, so the part was too skinny for the job, from a fatigue perspective, from the get go.

Where are you finding the lab reports on the Pickle Fork cracks?

It's rather hairy to note that of the now inspected 737 NG planes with over 30000 cycles, ~5% were found to have these cracks. By any measure, that is not a good result.

Yeah that's bad. But, damage tolerant design means these things usually don't cause airplanes to fall from the sky.  There will probably be a de-escalation of the inspection interval until the issue is better understood and the root cause figured out. I was involved in an investigation where the source of cracking in a part was never really totally resolved (long story) and a de-escalated inspection interval was considered sufficient risk mitigation and the new short interval stayed and that was that.

@DLH you'd have to be an employee of Boeing or the FAA to see that.  Those reports are not normally released to the public.

@JohnK:  you must be a Boeing employee then.

No retired, different OEM.

The latest findings of cracks in less than 20000 cycles (Lion Air, 2 planes), and 13% of cracked pickle forks in South Korean fleet are beginning to look really bad. This is a design flaw, no doubt about it.
As for how serious of a design flaw, I cannot judge. If the cracks would impose an immediate risk to flight safety, I bet the officials around the world would have grounded the whole fleet by now. Lets wait and see.


text:
Altimeter shows 100 foot drop when power is added. Is this a leak in the static system or broken altimeter and how long will the fix take?
My instructor and I were preparing for my Private Pilot Check ride today when we noticed that the analog altimeter would suddenly lose 100 feet when we added power.  The only way I can think to explain this is that there could be a leak in the static system that caused the altimeter to read the cabin pressure instead of outside static pressure.  We had the cabin vents open so that when the prop blew in more air it probably caused the cabin pressure to rise slightly causing the altimeter to read a lower altitude.  However, we only noticed an error with the altimeter and not with the airspeed or VSI.  At one point I was in a climb and the altimeter was showing my altitude decreasing and the VSI was showing an altitude increase.  If the problem was a leak in the static system you would think that the airspeed and VSI would be affected as well.  I did call the mechanic about this and he decided to take the airplane out of service.
My question is could this problem be isolated to just the altimeter and not the entire static system.  Also, what would the typical downtime be to fix this issue?  I am supposed to take my check ride Sunday but it looks like that may not happen if this would take awhile to fix.
Note:  This airplane does not have the option of using an alternative static source so no it is not due to the alternate static source being selected.

The mechanic will do a static system leak check.  This is done by applying a sealed off suction source to the static port, then applying an inch of suction, causing the altimeter to rise a thousand feet, then holding that for one minute (I do it using a big syringe connected to the blanked off static port on my homebuilt).  The altimeter is supposed to not move more than 100 feet in 1 minute (that's the rather generous Canadian requirement - not sure of the FAA but I assume it's similar)
If there are no obvious static leaks, and the static port itself seems normal, the mech will probably swap out the altimeter and see what happens.  Or, swap out the altimeter first and then try it out.
The leak check will take about 30 minutes, and the altimeter swap-out a couple hours.  Shouldn't take more than a morning to resolve.


text:
Why does the FAA not support UL102 (formerly 100SF) unleaded av-gas as an alternative to 100LL fuel?
The specifications for UL 102 are described in this Wikipedia article.  The fuel appears to exceed the specifications of 100LL in octane number and energy per unit volume although the fuel weighs 7 pounds per gallon instead of the 6 pounds per gallon for 100LL.  Is the additional weight the reason the FAA rejected this fuel as an 100LL alternative?

Technically, denser fuel is advantageous: in a burning mixture, the masses matter, not volumes. So you can take more fuel in the same tanks. (Weight/balance must be done by mass anyway, so those will be ensured). Whether it is compatible with existing injection and carburettor systems is another matter, and I guess this is the stumbling block...

The problem is basically bureaucratic delays, helped along by easing of EPA timetables on the banning of leaded fuel, taking some of the pressure off, which has caused the developer of UL102, Swift Fuels, to suspend its 2nd Phase testing until the machine gets moving.

@ John K:  I found a press release from Swift Fuels that says that Swift intentionally exited the PAFI program and sought out a different route for certification so I guess the fuel was not actually rejected by PAFI.  I did hear that PAFI was a bureaucratic nightmare and many manufactures refused to participate.  I had assumed the fuel was rejected because PAFI announced they rejected all submitted fuels but it look like UL102 is no longer part of PAFI.

AOPA article on unleaded fuel from Oct 2018.
https://www.aopa.org/news-and-media/all-news/2018/october/03/faa-sees-mid-2020-completion-of-unleaded-avgas-project

The FAA has re-evaluated its Piston Aviation Fuels Initiative schedule, reaffirming its commitment to develop a replacement for leaded avgas but delaying expected testing completion from December 2018 to mid-2020.
The new timeline focuses on testing of an unleaded fuel being developed by Shell—now the lone participant in PAFI following the elimination of Swift Fuels’ candidate fuel.

A more recent article, July 2019
https://www.aopa.org/news-and-media/all-news/2019/july/10/faa-issues-update-on-unleaded-avgas-research

A fuel under evaluation as a replacement for leaded avgas for the 170,000 aircraft in the piston-powered general aviation fleet will need “additional refinement,” and three new fuels have undergone preliminary testing in 2019, the FAA said in an update on its Piston Aviation Fuels Initiative.
According to the June 2019 program update, “PAFI's focus during the first 6 months of 2019 has included testing at the William J. Hughes Technical Center of an optimized Shell fuel and screening testing of 3 fuels not previously part of the PAFI program. The scope of PAFI has continued to evolve with the preliminary evaluation of 3 other fuels representing PAFI's commitment to research and evaluate all candidate unleaded fuels. Test results with the optimized Shell fuel were not successful with testing indicating additional refinements are required.”

AOPA's article appears to be an excerpt from this FAA article which has more details
https://www.faa.gov/about/initiatives/avgas/

My plane's a homebuilt so I just use mogas with no need for an STC.  I run some 100LL through it from time to time and also put some 100LL when it sits for the winter.  Modern mogas is amazingly clean.  I got a quart or so spilled on me when a friend dropped a gas can from the upper wing of his biplane and once it evaporated there was no residual odor on my shirt.  Even my wife couldn't tell it had gasoline on it.

100LL is the same re; evaporation.  How high  have you flown with Mogas? (which is no longer offered in many places, like my home field). What is the compression ratio on your cylinders? My engine is O360-A1F6D, 360 cubic inch, 180HP engine 8.5:1 compression ratio cylinders needs 100/100LL min per this Lycoming document:  Operator’s Manual Lycoming
O-360, HO-360, IO-360, AIO-360, HIO-360 & TIO-360 Series
Approved by FAA
8th Edition Part No. 60297-12
October 2005   https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO%20%26%20TIO-360%20Oper%20Manual%2060297-12.pdf

Up to 9000 ft..  My O-290 is 7.5:1.   In Canada the STC/STA has a 6000 ft restriction if you put it on a certified a/c, but not in the US.  The comp ratio's not that big a deal.  Believe it or not the STA in Can is available for various supercharged radials like the 985.  My glider club runs mogas in its O-540 Pawnee and 160HP O-320 Citabria.  Since your engine can scavenge the lead on 100LL and there are limitations to mogas and if you can afford the plane itself you can afford the gas, you are better off with 100LL anyway.


text:
Where to find information on the Daimler-Benz 606/610?
There seems to be relatively little information regarding either of these engines on the web. Anyone have an idea where I can find more info?

Point your browser to this page to find offers for the manual, spare parts list and illustrations for both engines. No free download though, you have to order a CD-ROM.


text:
Why is the MiG-23 landing gear nontraditional?
The MiG-23 has a very nontraditional landing gear setup. Why has it been designed the way it has as opposed to the more traditional method of a single compression strut that is raised into the fuselage.

(Source)

This configuration for the main landing gear was necessitated by the fact that the MiG-23/27's narrow fuselage combined with limited internal real estate in that section of the airframe made that type of retraction mechanism the best compromise in terms of fit, weight and providing a wide wheel track for the aircraft to facilitate operations on rough or unpaved airstrips (typical Soviet utilitarian philosophy for going to war).

An angled MLG strut which retracted forward such as that found on the F-104 or F-16 airplanes was not possible due to the side fuel tanks just aft of the engine inlets.
Aircraft which have narrow fuselages and high mounted wings are always going to require a more unorthodox retraction configuration for the main landing gear due to space limitations.  Other examples of this include the Dassault Mirage F1, Su-24 Fencer, and SEPECAT Jaguar.

This placement of the MLG and in this retraction configuration was the best available for the design limits.


text:
When was the Speed Trim System implemented on the Boeing 737?

The Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged.

Was the Speed Trim System implemented on the original Boeing 737 or just the Boeing 737 NG series?


text:
Can I still take the knowledge test with the US government shutdown?
Non-Essential employees at the FAA are out of work due to the US Government shutdown.  I am nearing the end of my Private Pilot Training and am almost ready to take the Knowledge test.  With the US Government shutdown, can I still take the knowledge test or do I have to wait for the government to reopen?

Is it administered via an automated online system these days? When I did mine back in the '90s, it was on paper in the FBO's office and mailed in for grading.

Looks like private testing centers can conduct the tests now.
Full details here, from 22 Sept 2017.
https://www.faa.gov/documentLibrary/media/Order/FAA_Order_8080.6H.pdf


text:
Is there a way to find what types of aircraft have landed at an airport?
When I was a child, I recall an aircraft landing at my local airport, CSG, I would like to know if airports keep records of what aircraft land there? If so how long do they keep these records, and are the available to the public?

Unless the flight is a historic one, typically the only data that gets recorded is the number of arrivals/departures based on aircraft weight/category. A lot of times these things are estimated and not actual values. So at least in the U.S., no record is kept that associates an aircraft registration number (specific aircraft) with arrivals and departures. Sometimes these are kept for airlines for billing purposes (landing fees), I don't know of any public source for that data though. See https://www.airnav.com/airport/KGRB for an example (scroll down to Operational Statistics).

@RonBeyer You could find data for a good number of flights into major airports on flight-tracking sites, those are only around for a day or two though since they get buried by the other flights coming in.

A lot of small airports do have a "guest book" that pilots will often sign. The ones I have seen usually only go back a few years, but old copies of the guest book are probably kept. Such records are entirely voluntary.

Are you talking about a type of aircraft or a specific plane? Something that landed there once or was it a recurring flight?

Simple answer... No. You could rule out the types of planes that could land at an airport by an airport's 5010 but there is nothing specific.

Speaking for the US, airports only report total movements (landings/takeoffs) and this information is available to the public via the FAA. This information is in aggregate and does not contain the type of aircraft.
Flightaware tracks only the type of aircrafts that land and takeoff at an airport; it also tracks the identifier used. It has very good coverage globally, although some areas have better (more accurate) information than others.
For most GA flights, the identifier is sometimes also the tail number, which can then be used to narrow down on a specific airframe.
Just add the ICAO identifier for your aerodome at the end of this URL: http://flightaware.com/live/airport/ to see stats of a particular aiport; for your airport its http://flightaware.com/live/airport/KCSG


text:
Is there a standard for required repainting of worn runway markings? In USA
Is there an FAA regulation or advisory circular covering the allowable wear, fading, or weathering of runway markings? I can find many FAA standards for the paint of fresh new runway markings, but I can not find any standard for determining when markings have become unacceptable due to wear or weather. It seems disconnected to create many specifications for new paint visibility but then to have no wear limits.
I am interested in federally funded or part 139 airports.(139 airports serve scheduled passenger service with more than 30 seats) I understand that private airports are not required to have standard markings.
There is a local public airport with center-line marks so old they cannot be easily seen from the traffic pattern, or final approach, even on a clear day.(the numbers and threshold bars are in good condition) I can still see the center-line paint when on the ground for takeoff and rollout. (no tower, but it does have a non-precision instrument approach and a dozen+ operations per hour in good weather)

What is the airfield? You might check in the US Chart Supplement for phrases such as “Markings: Non-Standard in poor condition” or “Markings: Non-Precision Instrument in good condition”. Many Class E & G airports will have that wording in their descriptions. Especially those that are non-towered. None of the airfields with non-standard markings will have precision IAPs. You might check the regulations for markings to see if they primarily apply to precision instrument approaches.

I don't want to know that the airport manager thinks the markings are "good"(the chart supp is mostly self reported info) I want to know if there is standard guidance for "good" condition. While unlikely, a runway with a precision IAP can have nonstandard markings if it is a private IAP. As opposed to a standard IAP developed for the public. It is unlikely because there is little reason to spend the effort to create such a combination. Restricted military runways with a P.IAP may also be allowed non standard markings.

Summary
No, there are no criteria to determine when markings require maintenance/repainting. Several elements exist:

AC 150/5340-1 and AC 150/5370-10 are the reference for markings design and construction.
AC 150/5200-18 and Order 5190.6 for inspection requirements.

But some are missing:

A measurement method. Research topic of AR-TN03/22 (2003).
Criteria for marking compliance. Were proposed in the research project.

The result is current checks for conspicuity are subjective, and there is, as far as I know and as surprising as it is, no predefined criteria to evaluate the need for maintenance. The research project conducted by FAA doesn't seem to have any regulatory implementation.
This project based evaluation on three criteria:

Retro-reflectivity threshold, depending on color.
Tolerance for tint change.
Paint coverage

No formal criteria for Part 139 commercial aviation, the most regulated, means no formal criteria exist for any type of airport in the US.

Part 139
Section 311 of Part 139, Marking, signs, and lighting, certainly requires markings to be "properly maintained" according to FAA "methods and procedures":

(a) Marking.
Each certificate holder must provide and maintain marking systems for air carrier operations on the airport that are authorized by the Administrator and consist of at least the following:
(1) Runway markings meeting the specifications for takeoff and landing minimums for each runway.
(2) A taxiway centerline.
(3) Taxiway edge markings, as appropriate.
(4) Holding position markings.
(5) Instrument landing system (ILS) critical area markings.
(d) Maintenance.
Each certificate holder must properly maintain each marking, sign, or lighting system installed and operated on the airport. As used in this section, to “properly maintain” includes cleaning, replacing, or repairing any faded, missing, or nonfunctional item; keeping each item unobscured and clearly visible; and ensuring that each item provides an accurate reference to the user.
(f) Standards.
FAA Advisory Circulars contain methods and procedures for the equipment, material, installation, and maintenance of marking, sign, and lighting systems listed in this section that are acceptable to the Administrator.

And also required related maintenance procedures to be described in the certification manual reviewed by FAA before granting its certificate:

A description of, and procedures for maintaining, the marking, signs, and lighting systems, as required under §139.311

FAA airport certification
FAA certifies airport according to Airport Compliance Manual — Order 5190.6. Its chapter 7, Airport Operations, determines responsibilities for the airport "sponsor" as related to maintenance for any certificated airport.
Part 139 airports will have additional constraints, not listed here:

7.1. Introduction.
[...] This chapter does not cover the additional requirements that Title 14 Code of Federal Regulations (CFR) Part 139, Certification of Airports, imposes on airports serving certificated scheduled air carriers. (Contact the FAA Airport Safety and Operations Division, AAS-300, in Washington DC, for additional information on Part 139 compliance matters.)

These sponsor responsibilities include runway inspection and repairs, included repainting markings:

7.2. Scope of Airport Maintenance Federal Obligations.
b. Airport Facilities to be Maintained.
[...] airport sponsors are required to inspect runways, taxiways, and other common-use paved areas at regular intervals to ensure compliance with operational and maintenance standards. Sponsors must make routine repairs, such as filling, sealing cracks, and repainting markings (as shown here) to prevent progressive pavement deterioration.

The actual way markings maintenance is conducted will be reviewed during the certification process. FAA provides guidelines in AC 150/5380-7A, Airport Pavement Management Program:

7.4. Maintenance Procedures.
Generally, airport agreements require the sponsor to carry out a continuing program of preventive and remedial maintenance. The maintenance program is intended to ensure that the airport facilities are at all times in good and serviceable condition to use in the way they were designed.
Advisory Circular (AC) 150/5380-7A, Airport Pavement Management Program, discusses the Airport Pavement Management System (APMS) concept, its essential components, and how it can be used to make cost-effective decisions about pavement maintenance and rehabilitation.

Self-inspections in part of the pavement management process are described in their own document.
Self inspection
Runway inspection, by airport owner/operator must comply with AC 150/5200-18, which references the marking specifications found in AC 150/5340-1 and states this standard is mandatory for Part 139, and when construction or rehabilitation is paid by federal funds:

c. Markings.
Airport markings provide important information to pilots during takeoff, landing, and taxiing.  To avoid confusion and disorientation, airport markings should be in compliance with FAA marking standards specified in AC 150/5340-1, Standards for Airport Markings. (Compliance with these standards is mandatory for operators of airports certificated under Part 139 and for airport operators that have accepted Federal funds for runway and taxiway construction/rehabilitation.)

Elements that "should" be taken into account during the inspection:

During the marking inspection, the inspector should:
(1) Check markings for correct color-coding, peeling, blistering, chipping, fading, and obscurity due to rubber buildup.
(2) Check to see if all runway hold position markings are clearly visible.
(3) During and after construction projects, check new markings for compliance with FAA marking standards.
(4) If the markings have glass beads, check markings during periods of darkness to determine if the reflectivity of glass beads is adequate at night.
(5) Report and monitor any nonstandard marking or markings that are obscured, faded or
deteriorating.

New marking measurements
There is no value indicated for marking visibility (conspicuity), but there are minimum values for different parameters given in AC 150/5370-10, Airport Construction Standards, at Item P-620 Runway and Taxiway Marking. They are used to evaluate the quality of the painting done by a contractor.
These criteria are numerous, due to the multiplicity of materials which can be used:

Waterborne, epoxy, methacrylate, solvent-base and preformed thermoplastic.
Inclusion of glass beads or not.
For waterborne or solvent based paints, different types (Federal Specification TT-P-1952F, types I, II, III) are used for different purposes.

Some values:

Daylight directional reflectance (relative to magnesium oxide, according to ASTM E2302) for epoxy paint: White: not less than 75% , Yellow: not less than 55%.
Retro-Reflectance for thermoplastic (mcd/m²/lux ASTM E1710): White 225, Yellow 100, Red 35.

Research by FAA
The FAA report for Development of Methods for Determining Airport Pavement Marking Effectiveness was released in 2003. From the abstract:

Presently, the conspicuity is determined by visual inspections of segments of these markings, but the validity of these inspections cannot always be confirmed.

This study was undertaken to develop a method for a quick and accurate evaluation of paint markings. A manual method was required for eliminating subjectivity in the current method, and an automated method was developed for evaluation of larger surface markings over a vast airport area. In addition, the study also established a threshold pass/fail limit for white and yellow paint.

It was found that for the manual method, three devices are required: (1) a retro-reflectometer is required for determining retro-reflectivity of the beads, (2) a spectrophotometer is required to determine whether or not the paint marking has faded out of tolerance, and (3) a transparent grid is used to determine coverage of the paint. If any one of these three tests fails, the pavement marking fails.

For the automated method, a van-mounted Laserlux or similar mobile unit is required. The automated method increases the speed and sample size. It works well for large airports, which have very long runway centerlines and threshold markings.

The retro-reflective threshold limit for yellow paint is 70 mcd/m²/lx and for white paint 100 mcd/m²/lx. The coverage threshold pass/fail limit is 50%.

ICAO
FAA indeed follows ICAO recommendations which are stated in Annex 14:

10.5.2 A system of preventive maintenance of visual aids shall be employed to ensure lighting and marking system reliability.

Note.— Guidance on preventive maintenance of visual aids is given in the Airport Services Manual (Doc 9137), Part 9.

Doc 9137:

8 MARKINGS
2.8.1 All markings on paved areas should be inspected at least semi-annually. Local conditions will determine when to inspect. In general, a spring and fall inspection will suffice to detect deterioration due to the winter and summer weather extremes.
2.8.2 Markings which are faded or discoloured by soil should be repainted. When rubber deposits have been removed from the pavement all defaced markings should be restored as soon as possible.

The main technical document for runways markings is IPRF 01-G-002-05-1, Airfield Marking Handbook. ICAO seems to use it as the reference for evaluating markings (example for Latin-America):

Faded colors or appearance.
Poor nighttime visibility or retro-reflectivity.
Existing markings are worn 50 percent or more.
Existing markings are covered with contaminants.
Markings are not representing properly its meaning anymore
Rust discoloration.
Algae growth.
UV-damage.
Positions and dimension of existing markings (tolerances).

I would hope that the research implicitly includes obscuration by dirt,(including tire rubber) within the category of paint coverage.

@MaxPower: This is not clear (see bottom of page 5). I'm quite surprised by the lack of actual standard, but maybe part 139 information that can be obtained by contacting FAA offers a better framework, even if not a standard or mandatory. Quite puzzling!


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Why don't airports have weigh stations to prevent overweight takeoffs?
Currently, ensuring that an airplane isn't overweight when it takes off relies on the cargo handlers loading the aircraft properly and accurately documenting (on the weight-and-balance sheet) what with how much weight was placed where, and on the pilots properly reviewing the weight-and-balance sheet once the airplane is loaded.  Even then, there are uncertainties (it is rarely possible to know exactly how much each passenger weighs, for instance).  So why don't airports have weigh stations to weigh the planes as they taxi after being loaded, allowing any flights found to be overweight to be ordered back to the terminal to jettison the excess weight?

Some airplanes have such devices and the scales you describe exist they are just not used for every flight.

Is this even a practical problem that needs solving? Aren't most flights loaded (usually fuel-wise) under max take off weight anyway? Airlines don't like to tanker fuel all around unnecessarily.

Might it not be simpler to build the scales into the landing gear?

Aircraft are heavy.
Really, quite heavy.
An Airbus A340, depending on type, has a maximum takeoff weight (MTOW) of up to 380,000 kg.
A Boeing 747, again depending on type, can have a maximum takeoff weight of a shade under 447,700 kg.
Measuring that kind of weight accurately is certainly not impossible, but it's not exactly easy either.
A bigger issue is that it's not as simple as saying that if the aircraft weighs more than some number of kilograms, then it's overweight, and if it weighs less, then all is fine. The distribution of that weight within the aircraft – precisely that "weight and balance calculation" which you mention – matters, sometimes a great deal. The aircraft could be putting an acceptable weight on the wheels, but if the weight is too far forward or too far aft, may still be outside of its controllability limits once it lifts off the ground.
Simply putting the aircraft on a scale and providing the pilots (somehow) with a weight-on-wheels readout as they taxi will catch the situation where the pilot can't add (as in, mathematically add up the weight components that make up the total aircraft weight), but it won't necessarily catch the situation where the load is improperly balanced.
So you'd be adding another fair bit of complexity (which can fail in various ways) to a currently relatively simple part of aviation (the taxiways or apron), possibly giving the pilots a false sense of security, and not solve the problem. Someone would still need to do a weight and balance calculation, at which point the weight-on-wheels reading can, at best, provide a rough sanity check on the weight part of the weight and balance.
Seems easier to just try to figure out ways to make the weight and balance calculations easier to perform, and make random spot checks on weight and balance calculations done (and schedule followup training if the pilots, or ground crew, or whoever else, need it).

Plus, for cargo aircraft, there are considerations which just knowing the weight on the individual wheels is not going to resolve. A freighter could be within the overall weight and balance envelope for the airplane and still be violating monocoque (cumulative loading so to speak from a given point forward and aft) limits, linear limits (floor loading in a smaller area so to speak), and for some aircraft the vertical c.g. If you want, to get an idea of the complexity, go to 747.terryliittschwager.com. Use a 767 if you want the added complexity of vertical c.g. limits.

Thinking about it, it shouldn't be too hard to determine the longitudinal location of the center of mass, simply by putting the nosegear and the main gear on separate weighing platforms and measuring what proportion of the weight rests on what gear.  @Terry: Sure, it'd be a lot more complicated for freighters, but passenger aircraft aren't likely to experience those sort of problems.

@Sean You are correct that longitudinal c.g. can be handled by knowing the weight on each landing gear, and the lateral c.g. as well. There are some aircraft that have that built in. Insofar as the distinction between pax aircraft and freighters, remember that these days airlines are carrying cargo in the lower holds, and those holds are subject to linear and cumulative limits, not to mention ULD limits (Unit Load Devices), the pallets and igloos the cargo is in.

I would think you could calculate the CG based on the loading.  Ie what % of the total is on the nose wheel vs the mains and calculate that automatically based on aircraft type data.

It's theoretically possible, but an airport would have to put in weighing pads for each landing gear, that can accommodate all sizes of airliners, and it would have to be in a specific spot for the purpose, which would require everybody to taxi to the same spot to get weighed causing a huge bottleneck (deicing pans are bad enough).
And the airport authority would have to pay for it, when there is nothing in it for them, and airlines wouldn't pay the weighing fees in the first place.  And the current methods work fine.
In essence it's a solution in search of a problem.

The weighing area should be inside a building or hangar so the effects of the wind could be eliminated.

Good point.  There ya go.  Even worse.

Airports could just defer the costs to the airlines as part of their pre-existing fees.  Airlines would not have a choice.  Airports have the motivation of reducing crashes, especially those which are likely to occur very close to the airport (typical for overweight conditions), and possibly shutting it down depending on the crash site.

Crashes of major airliners due to taking off overweight or with CGs out of limits is close to nonexistent in the West.  A solution in search of a problem, with zero need in the practical world.

There is also economics to consider.
The number of airliner crashes that result from overweight or weight imbalance takeoffs is very low, and almost always confined to marginal operators trying to pack in more paying customers, or not paying attention to weight balance.
Takeoff crashes most often occur when there is too much ice on the wings (Air Florida 90), or the flaps aren't set correctly (Northwest 255), or there is a problem with the landing gear dragging (Lokomotiv Yaroslavl, Arrow Air), none of which is directly related to overweight conditions.
Checking weight on every takeoff would be extremely expensive to implement, would impose additional delays on an industry already wrestling with delays, and wouldn't result in a big increase in safety, because very few airliners crash due to weight problems.
A more efficient course to follow is to continue bearing down on marginal operators, and solve a lot of safety issues, overweight and otherwise, since the bulk of air crashes in general seem to originate with them. More crashes will be averted for less money.

The OP needs to research AF90.  The problem was not too much ice on the wings!  It was iced over sensors in the engine.  The flight crew thought they had selected take-off thrust but instead had selected significantly less, and the plane with whatever ice load it did have couldn’t fly with that little thrust.  OP, you need to list crashes caused by overweight aircraft to convince us that there is a problem here...  nobody wants to pay for a solution in search of a problem.  Convince us, please, that there is a problem here.  And thanks in advance for your research on this matter.


text:
If Floyd Bennett Field failed due to its remoteness and isolation, how has JFK been so successful?
Floyd Bennett Field was built at the end of the 1920s in order to give New York City its own airport and avoid having to rely on Newark; however, despite its superb airport facilities, Floyd Bennett Field was never a commercial success, being cursed by the relative remoteness and isolation (compared to Newark) of its location in far southeastern Brooklyn.  As a result, when the airlines and the airmail stayed at Newark, LaGuardia Airport was built in northern Queens, quickly becoming much more successful than Floyd Bennett ever was, and remaining New York’s primary airport until 1948, when Idlewild Airport (JFK from 1963) opened in far southeastern Queens.  Idlewild/JFK, in turn, quickly came to overshadow both LaGuardia and Newark, and has been the primary airport of the New York metropolis ever since.
But JFK is even more remote than Floyd Bennett!

If remoteness killed Floyd Bennett, why hasn’t it killed JFK?

it might help to put a map scale on your picture.  All three sites are less than 20 miles apart, and Newark is just off to the left past Staten Island.  May have been politics.  Also Idlewild/Kennedy was simply in the right place at the right time as both Long Island and aviation boomed.


text:
Was the foil in older FDRs reused when it ran out, or replaced?
Before magnetic tape became the standard recording medium for flight data recorders, most FDRs used a long strip of metal foil that had altitude, airspeed, etc. traces scratched into it by a set of styli inside the FDR (although a few very early FDRs used wire recording instead).  Judging from a number of NTSB accident reports that mention FDRs where the foil had run out and been taped down at the end for maintenance, this presumably wasn’t a continuous loop of foil (although I can’t imagine it would have been hard to implement a continuous-loop system; it seems to me that, to make such a system workable, all you would have had to do would have been to squeeze the foil through a set of metal lips to flatten out the old traces just prior to passing under the sticks), but, instead, one that had to be removed and dealt with by maintenance when it ran out.  When a foil FDR became full and was removed for maintenance, were the traces flattened out of the foil and the foil reused, or was the strip of foil replaced with a new, empty strip (with the full strip presumably being either discarded or possibly stored in an archive somewhere)?

Those older FDR's are known as "foil oscillographic recorders." This technology is similar to the old recording barometers, which were used for, among other things, providing evidence of altitude in glider record-attempt flights. Just as in these barometers, the recording surface was not reusable in those FDR's. When it would run out, it needed to be changed. The FDR's had an indicator which showed how much usable foil was left in terms of hours. The rolls of foil were 200 ft long and ran at 6 inches per hour when the aircraft was operating, about 400 hours' worth. This was an inspection item on the aircraft.
In addition, the foil was made of Inconel, which is known as a "superalloy" that is resistant to very high temperatures. It would be very difficult to remove the tracings of data without specialised equipment, and in reality the tracings removed material from the protective surface coating on the foil. If you recycled it the quality of the data could be impacted by corrosion.
This very cool video illustrates how these FDRs worked.


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Why are the throttle levers for the Comet's outboard engines shaped funny?
The throttle levers for the Comet's outboard (#1 and #4) engines are weirdly shaped, with flanges that extend up and partially over the throttle levers for the inboard (#2 and #3) engines:

(Image cropped and annotated by me; original by Geni at Wikimedia Commons)
Why are the Comet's outboard throttle levers shaped like this?

Sean, this will probably surprise you as much as it did me, but the answer is that it is intended to help keep the hand of the pilot operating the throttles on them and not have it slip off. In my opinion, it's a nifty little design detail.

Which Comet/Nimrod-series aircraft is that photo from?

A Comet 4 built in 1958. The photo is a screen capture from this very interesting video on YouTube. https://www.youtube.com/watch?v=nuOpFjxmP4c. I am not sure the panel is completely original but the throttle quadrant appears to be the original and the design of the throttle quadrant appears to be identical on all Comets.

Huh.  The instrument layout of that Comet 4 looks considerably different from that of the Comet 4 in the image from which I cropped the image in my question...  EDIT: I see you edited your comment while I was writing mine!  :-P

The video shows portions of the instrument panel and instruments restoration.